1. Field of the Invention
This invention relates to an improved construction for composite building boards. More particularly, the present invention relates to a composite building board with an external hot melt thermoplastic coating applied over a cementitious pre-coated fibrous mat.
2. Description of the Background Art
Building board, also known as wallboard, plasterboard, or drywall, is one of the most commonly used building components in the world today. Building board is frequently used within the interior of a dwelling, where it functions both as a finished wall covering and as a structural room partition. Building board can also be used on the exterior of a dwelling, where it serves as a sheathing to provide weather protection and insulation. Building board can also be used as an interior facing for other structures as well, such as stairwells, elevator shafts, and interior ducting.
One particularly popular form of building board is known as gypsum board. Gypsum board is constructed by depositing a layer of cementitious gypsum slurry between two opposing paper liners. Gypsum slurry is the semi-hydrous form of calcium sulfate and has many physical characteristics that make it suitable for use as a building component. For example, gypsum boards generally have a smooth paper surface, a consistent thickness, and allow for the application of finishing enhancements, such as paint. Gypsum board is also desirable because it provides a degree of fire resistance and sound abatement.
An example of a paper-covered gypsum board is disclosed in U.S. Pat. No. 2,806,811 to Von Hazmburg. Von Hazmburg discloses a board that primarily consists of a thick gypsum core that is encased in a fibrous envelope consisting of both a manila sheet and a newsprint sheet. These sheet layers can be made from a conventional multi-cylinder paper making process.
Although conventional paper faced gypsum board, such as that disclosed by Von Hazmburg, is acceptable for many applications, it also has considerable drawbacks. A major drawback is durability. Gypsum board is far more brittle than other building materials, such as wood or masonry based materials. Paper faced gypsum boards, therefore, chip and/or crumble under both compressive and tensile loads. As a result, conventional gypsum board is easily damaged during normal wear and tear within a dwelling, such as impacts with people and/or furniture. Conventional gypsum board assemblies often have low load carrying capacity and inadequate nail pull strength. As a result, traditional gypsum board often cannot support the loads needed to hang pictures or install shelving without the use of supplemental fasteners.
As a consequence of these drawbacks, efforts have been made over the years to improve the durability and surface strength of gypsum board. One particularly useful development is known as glass reinforced gypsum (GRG) board. An example of one such board is disclosed in U.S. Pat. No. 4,265,979 to Baehr et. al. Baehr discloses a paper-free gypsum board construction. More specifically, Baehr replaces paper facing sheets with opposing layers formed, in part, from glass fiber mats. This construction provides a stronger and harder external surface and is an improvement over paper faced boards. Although an improvement from the standpoint of durability, the use of exposed fiber mats is problematic. Namely, workers handling such boards are exposed to lose strands of fiber. This poses a health risk and necessitates the use of protective gloves and/or masks. Thus, GRG boards utilizing exposed facing sheets are not ideal.
A subsequent improvement is described in commonly owned U.S. Pat. No. 4,378,405 to Pilgrim. The contents of the Pilgrim patent are fully incorporated herein by reference. Pilgrim discloses a GRG board that is faced on one or both sides with a porous, nonwoven glass mat. However, the glass mat of Pilgrim is slightly embedded into the slurry core. This is accomplished by vibrating the gypsum slurry to cause it to pass through the porous openings in the mat.
Embedding the mat within the core results in a thin film of slurry being formed on the outer surface of the board. Building boards with this construction are referred to as embedded glass reinforced gypsum (EGRG) boards. EGRG boards eliminate, or greatly reduce, the presence of exposed fibers and otherwise provide a smooth working surface. Despite eliminating the safety issues surrounding GRG boards, Pilgrim ultimately failed to provide a board with sufficient strength and durability.
A further improved EGRG board is disclosed in commonly owned U.S. Pat. No. 6,524,679 to Hauber, et al. The contents of the Hauber patent are fully incorporated herein by reference. The EGRG board of Hauber adds a polymeric compound to the gypsum slurry. Suitable polymeric compounds may include, for example, polyvinyidene chloride (PVDC), or polyvinylchloride (PVC), or similar polymers. The polymer additive increases durability and board strength and also creates a matrix within the slurry after it sets. Although certainly an improvement over existing EGRG technology, Hauber did not address issues associated with the durability of the exterior face or the complete mechanical and chemical bonding of the exterior face to the underlying gypsum slurry.
Thus, there still exists a need in the art for improved building board construction. More specifically, there is a need in the art for a board with a polymer matrix that provides enhanced durability, impact resistance, water repellency, fire resistance, and load carrying capacities. There is also a need in the art for a board that provides these physical properties without unduly increasing the weight or cost of the resulting board. The present invention is aimed at achieving these objectives.